CONTAMINATION OF MARS
Carl Sagan
Elliott C. Levinthal
and
Joshua Lederberg
June 1967
Smithsonian Institution
Astrophysical Observatory
Cambridge, Massachusetts, 02138
Contamination of Mars
Carl Sagan, Elliott C. Levinthal, Joshua Lederberg
TWO papers concerning the problem of biological contamination of Mars
have recently appeared in this journal. The first (9 argues that the proba-
bility of release of entrapped organisms from the inside of a spacecraft is
vanishingly small, and that the probability of reproduction of a terrestrial
microbial contaminant on the Martian surface is also negligibly small. The
second paper (z) argues that in any case Mars (and also Venus) may have
been exposed to terrestrial microorganisms deposited by Soviet spacecraft.
Both papers conclude that the present sterilization standards adopted by the
Committee on Space Research (COSPAR) of the International Council of
SC ientific Unions be abandoned. These recommendations are that the prob-
ability of a single viable organism aboard any spacecraft intended for
Martian landing be less than 1 X 10 -4
, and that the probability for accidental
impact over a period of decades by each unsterilized flyby or orbiter be
3 x 1o-5 or less. We have very grave reservations about the arguments
presented in both papers, and wish here to clarify the difference between
their views and ours, and to delineate problem areas crying out for fur-
the r study.
Dr. Sagan is Assistant Professor of Astronomy at Harvard University and
staff member of the Smithsonian Astrophysical Observatory, Cambridge,
Massachusetts. Drs. Levinthal and Lederberg are, respectively, Director,
Instrumentation Research Laboratory, and Professor of Genetics, Depart-
ment of Genetics, Stanford University School of Medicine, Palo Alto,
California.
The COSPAR recommendations were based upon an analytic equation for
r, the number of viable microorganisms per capsule deposited on the Martian
surface, obtained from probability theory by Sagan and Coleman [(J); slightly
revised in (i)] . The focus of this discussion was the desirability of perform-
ing a large number of biological experiments on the Martian surface before
there is a sizable probability of contamination. (It can also be argued that
more emphasis should be placed on the risks of the very first missions, since
these may help set wider goals than the biological exploration of Mars. ) Our
commitment to this goal, measured by the parameter p, was put at 0. 999.
While the discussion of Sagan and Coleman inserted representative numerical
values into the analytic expression, it was indicated that these were tentative
values, subject to revision by future research, and the reader was invited to
make his own selection of numerical values. The COSPAR deliberations on
acceptable levels of contamination occurred in Florence in May 1964, in a
study group on standards for space probe sterilization, chaired by Professor
C. -G. Heden, of the Karolinska Institute, Stockholm. The other members
of this group were Alan Brown, U. S. A. ; Auduoin Dollfus, France; Marcel
Florkin, Belgium; Lawrence Hall, U. S. A. ; A. A. Imshenetskii, U. S. S. R. ;
Carl Sagan, U. S. A. ; P. H. A. Sneath, United Kingdom; and Wolf Vishniac,
U. S. A. In the deliberations of this study group each member present (all
but Professor Imshenetskii, who was absent because of illness) gave his best
estimate for each numerical parameter entering into the analytic framework.
The resulting values of (r were then derived. The discussion iterated in a
-4
final value of u = 10 . Contrary to the assertion in (l), the numerical
-
values adopted by Sagan and Coleman were not in all cases those adopted by
the COSPAR group, and the value of 10 -4 adopted by the study group, and
approved by the executive council of COSPAR, represented the best opinion
of the study group menbers. Subsequent COSPAR deliberations have reduced
-3
uto10 .
In the absence of a large body of experience with Mariner and Voyager
spacecraft, and in the absence of in situ studies of the Martian surface and
subsurface, we are very ignorant of the relevant parameters. A single
terrestrial microorganism reproducing as slowly as once a month on Mars
2
would, in the absence of other ecological limitations, result in less than a
decade in a microbial population of the Martian soil comparable to that of
the Earth's. This is an example of heuristic interest only, but it does
indicate that the errors in problems of planetary contamination may be
extremely serious. If we agree that we desire 1 - p = 10 -3 with an uncertainty
in our commitment of, say, a factor of 100, then each other parameter of
relevance, such as the probability of release of encapsuled contaminants, or
the probability of multiplication of terrestrial microorganisms on Mars,
must be known to roughly similar precision. For each of the objections posed
by Horowitz et al. (L), we believe there exist uncertainties in the relevant
--
probabilities of factors of 100 or more, to say nothing of the possible presence
of unforseen factors. Where there are legitimate differences of opinion in
discussions of planetary quarantine, the burden of proof must fall on those
advocating a relaxation of standards. Horowitz et al. state that a large
--
upward revision of the allowable microbial load per spacecraft is now pos-
sible, on the basis of new evidence. We now examine this evidence. It
falls into four categories, release, survival, dissemination, and growth of
terrestrial microbial contaminants on Mars.
Release
.' -
An important point in the connection between the COSPAR conclusions
and the arguments presented in (3) and (4) appears to have been missed by
- -
the authors of (1).
- The parameter cr is defined as "the mean number of
viable microorganisms per capsule which are distributed outside the cap-
sule, on the Martian surface" [(A), italics in original] . The C OSPAR
recommendations refer to the "probability that a single viable organism
be aboard any vehicle intended for planetary landing" [Q), italics added].
The difference clearly relates to the mean probability of release of a con-
tained microorganism. The COSPAR deliberations and the discussion of
(3) and (4) were influenced by our ignorance of release over a period
- -
of decades. Horowitz et al. argue that we can now decide that this release
--
probability is vanishingly small, and that surface sterilization techniques
should therefore be adequate. The principal difference between our approach
3
and theirs is our unwillingness to consider a compound risk that may be
small by conventional standards - say, 10 -2 - as equivalent to zero when
the stakes are very high.
One serious contingency for release of contained microorganisms is a
crash-landing, and, particularly, a hypervelocity impact. Judging from
experience with lunar cratering, a spacecraft impacting Mars with a velocity
about 6 km/set will be totally pulverized. But experience with missile
impact indicates that even at impact velocities of 0. 6 km/set or less, a
significant fraction of the missile's mass is not in the impact crater and is
unrecoverable (6). The shells and grenades used in bacteriological warfare
indicate that contained microorganisms will survive such impacts, The
escape velocity from Mars is 5 km set -1
. Capsules intended for entry
into the Martian atmosphere can be designed so that aerodynamic braking
will slow the vehicle down to a velocity << 1 km/set with some high reli-
ability, and for a range of such contingencies as the approach orientation
of the capsule (1). But what the probabilities actually are is not well known.
There are other contingencies such as an unfavorable retrorocket firing
accelerating rather than decelerating the capsule, accidental failure of a
bus-deflection maneuver, etc. A specification of each contingency of
hypervelocity impact with associated probabilities needs to be made; until
then probabilities - 10 -2 of hypervelocity impact are not obviously unreason-
able for planned U. S. and Soviet missions. This probability- can be made
<< 10 -2 by failsafe precautionary terminal corrections in case of imminent
high- speed impact, or by the development of failsafe terminal-destruct
sterilization procedures (2,+); but such devices are not now planned in the
Mariner and Voyager programs.
Information on the fragmentation size distribution of lower velocity
crash-landings is needed and almost totally lacking. This should be done for
a variety of impact velocities, and the possibility that the fragmentation
distribution function is bi- or polymodal should not be overlooked. If frag-
mentation tends to occur along identifiable fracture planes, surface sterili-
zation of such planes will be very effective, as Horowitz (8) has emphasized.
-
4
Unfortunately there are almost no data available on fracture modes for
various scenarios of mission failure. An engineering examination of this
subject is urgently needed.
Even after the successful landing of an intact spacecraft, there is a
variety of possible release mechanisms. It is argued in (1) that aeolian
-
erosion is negligible for a period of decades on Mars. One argument is that
calculations by Leovy (9) put the maximum wind velocity on Mars as 80 to
-
160 km/hr, while the authors calculate that 145 to 250 km/hr is required
for rolling and saltation. Horowitz et al. conclude from this that the Martian
--
winds are "probably too low to initiate grain movement, I' (our italics).
Both calculations are, in fact, so uncertain that the results can more accu-
rately be considered identical. The large temperature fluctuations on Mars
lead naturally to the presence of dust devils with large vortex velocities
(10) and frequent observations of yellow clouds with the same photometric
-
properties as the Martian bright areas strongly point to windblown dust (11).
-
The idea that all, or even most, of the dust storms are produced by meteorite
impacts is highly unlikely, and contrary to the opinion of the majority of
observers on Mars. The Mariner 4 photographs show a very marked filling
of Martian craters, probably by dust. Martian craters are generally much
more eroded, and have much flatter bottoms and more filled-in appearances,
than craters on the Moon.
Horowitz et al. quote aeolian erosion rates in lucite for terrestrial
--
deserts of just under 1 mm every few decades. While lucite is a relatively
soft material, the extrapolation of these results to the erosion of exterior
spacecraft components on Mars is surely uncertain to several orders of
magnitude. Further information on expected aeolian erosion rates on Mars
is badly needed. There are many prospective components of Mars landing
vehicles that have structure and crevices at a depth of some millimeters,
and it appears obvious that, for the present, sterilization at least down to
such depths is necessary. Surface sterilization may be inadequate for this task.
Heat soaking is probably required, but for this purpose lower temperatures
or shorter times might be adequate: the thermal wave need not penetrate to
the deep interior of the spacecraft.
5
It must also not be forgotten that the values of parameters chosen in (2)
and (4) represent averages over many missions. Our immediate concern is
-
with the choice of parameters for the first landed missions, and here we
must be more cautious. The chance of an accidental crash landing is prob-
ably not less than 0. 1, but might optimistically be made as small as 0. 5.
Even this number in the arguments presented in (1) fails to take into account
-
the large range of intermediate possibilities between complete failure due to
crash landing, and aeolian erosion down to a depth of some millimeters.
For example, 1 -P, is not the chance of a catastrophic crash landing as
stated in (l), but rather the probability that some fraction of the experiments
-
in a lander do not operate as planned. Pt is the probability of complete
engineering and scientific success. We must take into account the possibility
of partial damage, which allows organisms from some parts of the space-
craft but not from others to reach the Martian surface.
There are several other spacecraft fractionation mechanisms that have
not been given nearly enough attention. The high prevailing winds, both diur-
nal and seasonal (the seasonal thermal and geostrophic winds are probably even
higher than the diurnal ones) will produce vibrational stresses over long
periods. There is possible metal fatigue and thermal erosion due to the
very large diurnal and seasonal temperature variations. While there is some
data on rocks, there is no information on the survival of spacecraft after the
14,000 cycles over 100 K" represented by a few decades on Mars. Since
spacecraft are constructed to survive the thermal environment of space
during transit, they probably can be constructed to survive a few decades on
Mars. But there is no evidence that they are being so constructed. There
are possibilities of chemical weathering. The oxide coats of surface metals
(thickness, a few microns) will certaintly be breached by aeolian erosion.
If the Martian environment is slightly reducing (e. g., if halide-substituted
hydrocarbons are present) chemical weathering of the exposed metallic sur-
faces will occur. While many nonaqueous chemical corrosives are also
sterilants we have no assurance that all organisms released by chemical
weathering will automatically be killed. There may even be erosion
6
mechanisms involving an indigenous biota. There may well be other space-
craft erosion mechanisms that we have not been clever enough to think of.
In order to allow for such possibilities, the interiors of spacecraft must be
sterilized until better information is available.
Horowitz et al. do not question experiments that show that many varie-
--
ties of terrestrial microorganisms survive simulated Martian conditions
indefinitely. But legitimate questions can be raised about what fraction
of the likely microbial contaminants of a spacecraft can survive Martian
conditions [cf. (2, z)] . Most such contaminants are probably of human
origin or derived during assembly and storage at the launch facility (generally
in warm climates for the United States, in colder climates for the Soviet
Union). Do the likely contaminants include anaerobic psychrophils? Are
those organisms that survive terminal heat soaking likely to survive
Martian low temperatures ? What fraction of released contaminants will be
killed on Mars by ultraviolet light before being shielded by absorbing grains
of surface material, or by other microorganisms. 3 Each of these questions
can be approached experimentally; until they have, we must proceed
cautiously .
Dissemination
In the case of a hard landing, and, particularly, of a hypervelocity
impact of a spacecraft on Mars, spacecraft fragments will be distributed
over a wide area. In a period of minutes - less than the time for many
unshielded terrestrial organisms to accumulate an ultraviolet mean lethal
dose on Mars - fragments travelling in parabolic trajectories at 6 km/set
will cover a lateral distance N 1000 km. A typical microbial load of a
surface-sterilized spacecraft assembled under clean room procedures is
107, distributed uniformly over the Martian surface, it results in a loading
density of one microorganism every 10 km2 on Mars. However, there
7
are failure modes that lead to much higher loading densities: e. g., inflight
rupture of nutrient broth containers carried for bioassays on Mars. Space-
craft should be designed to minimize the risks attendant to such failures.
Aside from crash-landings there are other possible dissemination mech-
anisms. The prominent dust storms and high wind velocities previously
referred to imply that aerial transport of contaminants will occur on Mars.
While it is probably true that a single unshielded terrestrial microorganism
on the Martian surface - even the most radiation-resistant variety - would
rapidly be enervated and killed by the ultraviolet flux, this by no means
applies to all contamination scenarios. The Martian surface material cer-
tainly contains a substantial fraction of ferric oxides, which are extremely
strongly absorbing in the near ultraviolet. In fact, apart from the ferric
oxide identification, the red color of Mars clearly indicates major electronic
transitions at short visible wavelengths. A terrestrial microorganism im-
bedded in such a particle can be shielded from ultraviolet light and still be
transported about the planet. In addition, microorganisms generally do not
distribute themselves singly, but tend to clump. While the peripheral organ-
isms may be rapidly killed by solar ultraviolet radiation, microorganisms in
the clump interiors may survive for long periods of time. These points have
experimental confirmation, in the Luster program of spaceflights in the vicinity
of the Earth (12).
- Here microorganisms were observed to survive solar
ultraviolet light and other radiation for much longer than the time calculated
on the basis of individual death curves. Not all released contaminants will
successfully adhere to shielding particles. There are simple laboratory
experiments on the ultraviolet illumination of an aerosol of bacteria and
pulverized limonite, and subsequent scoring, which can clarify these issues.
We do not know enough about Mars to exclude subsurface transportation
of contaminants - e. g., by underground rivers. Terrestrial volcanic belts
are connected by such river systems, and some evidence exists for tectonic
activity on Mars (see below).
8
If aqueous environments exist on Mars, we shall undoubtedly bias our
landing areas to such sites. The resulting microbial load can thereby be
amplified by many orders of magnitude; and if aqueous underground transport
is a real possibility, dissemination will also be amplified. In either case,
mean contaminant loading densities of 1 m -2 and much higher densities in the
biologically most interesting areas are by no means out of the question.
Growth
Before we discuss the problem of growth of contaminants on Mars, we
wish to stress that the dissemination of viable microorganisms without growth
may have serious consequences. One result of continuing observations could
be an assessment of the potential future utility of the planet Mars by the
human species. No such assessment has thus far been made, and such con-
siderations have had, to date, no effect on our level of commitment to a
decontamination program. It is obvious that if the future utility of Mars were
thought to be great, we would be willing to increase the costs and efforts to
achieve effective sterilization. Any future exploitation of Mars might be
seriously compromised by the dissemination even of nonpropagating terres-
trial microorganisms in the form of spores that survive the present environ-
ment.
The question of growth of terrestrial microorganisms on Mars is tied
to the question of the availability of liquid water. While the mean Martian
conditions are inconsistent with liquid water, there are a variety of possible
microenvironments that permit liquid water, at least in isolated times and
places. We have suggested (13) that geothermal activity on Mars provides
-
water and local high temperatures conducive for the replication of terres-
trial-type organisms. Horowitz et al. (1) believe that they know enough about
--
the Martian environment to exclude such possibilities, and state their belief
that Mars is an undifferentiated and geologically inactive planet. They also
state that, while there may be lineaments observed in the Mariner 4 photo-
graphs, on the Earth the water outgassing associated with such lineaments
is recirculated surface water, and not juvenile water.
9
However, Mars very likely contains a subsurface permafrost layer,
and there is now a variety of evidence for loosely bound or adsorbed water
in the surface material (14, 15). In either case geothermal activity would
--
release otherwise unavailable water to the surface. At any given time the
fraction of the planet undergoing such geothermal activity should be very
small, how ever. It is also possible that the permafrost cap is breached
occasionally by meteorite impact. The question of the differentiation of
Mars is a highly debatable one. Meteorites whose parent body radii are far
smaller than Mars show clear signs of differentiation. The Moon, with a
mass much less than that of Mars, shows unambiguous signs of vulcanism.
There are also several cases of lunar outgassing, apparently well documented.
Studies of the thermal history of Mars are in a preliminary state, and at
least some models (16) predict a fairly differentiated planet. Radar and
-
other evidence for major elevation differences on Mars, and ridges resem-
bling submarine tectonic ridges on Earth, has been published (17, 18).
-
The lifetime of liquid water on Mars is a more serious question. Salt
deposits will lower the eutectic point - in many cases by several tens of
degrees. There may be frequent briney pools on Mars. Horowitz et al.
maintain that all halophiles are aerobes, but we believe this must be an arti-
fact of the experimental conditions; not much effort has been put into finding
anaerobic halophile s ( 19).
- The triple point partial pressure of pure liquid
water is just under 6 mb. Average total pressures on Mars probably range
from a few mb to almost 20 mb (18), but the mixing ratio of water is - 10 -3
.
-
Equatorial daytime temperatures range up to 20 or 30" C (3, corresponding
to saturation vapor pressures of 25 mb and higher. A pool of liquid water
exposed on Mars at temperatures 1 273" K will evaporate initially at a rate
- 1o-2 2
g/cm /sec. The evaporation is so fast that vertical and horizon-
tal transport of water away from the pool will be the rate-limiting steps
in the early phases of vaporization. Because the saturation vapor pressure
at these temperatures is of the same order as the total pressure the pressure
difference will lead to hydrodynamic flow, and winds will carry the vapor
away. The transport rate has not, to the best of our knowledge, been cal-
culated. Before long, however, the high latent heat of vaporization of water
10
should result in freezing the upper surface of the pool, cutting down the
vaporization rate substantially. It may be that some pools on Mars are thus
exposed during the hottest part of the day, and sealed for the remainder of
the day.
Other locales where the partial pressure of water may temporarily be
comparable to the total ambient pressure are the edge of the polar caps, and
the dawn limb of Mars. Because of the inhibition of horizontal eddy diffusion,
a pool at the bottom of a crater (perhaps formed during crater formation) is
of particular interest. The frost point of 20 TV precipitable water vapor is
about 190" K, a temperature reached before sunrise, probably in most
locales on Mars. A dawn haze is actually seen frequently. After sunrise
the computed lifetimes of the condensates are 15 min or less. However,
diffusion will again be the limiting step, and saturation vapor pressures may
be maintained in crevices and soil interstices for periods approaching an
hour. As the temperature goes above 0" C, liquid water will be formed
temporarily. Experiments performed in the laboratory of one of us (C. S. )
under Martian pressures and temperature cycling, and with a variety of wind
velocities, tend to support the conclusion that thin layers of liquid water are
generally formed each morning at tropical latitudes on Mars. These experi-
ments will be reported more fully elsewhere. A variety of experiments (21)
-
indicates that many terrestrial microorganisms can grow, even if liquid
water is available for only 15 min/day, in otherwise subzero condi-
tions; or in soil microenvironments where liquid water is available as a con-
sequence of freeze-thaw cycling. Although much more work is required,
many microorganisms seen capable of growth at slow rates at temperatures
near 0" C. There are too many possibilities still open to dismiss significant
growth and replication of terrestrial microorganisms on Mars, over a per-
iod of decades.
The use of 10 -2 for the probability that a terrestrial microorganism
deposited on the surface of Mars will grow and contaminate the planet may
in fact be too low rather than too high. We anticipate that we will be clever
in planning the strategy of planetary exploration; that is, over a long time
scale we will land not just at an average location on the planet, but at a place
11
where terrestrial microorganisms are most likely to grow. Our subsequent
concern is not necessarily with growth over large areas of the planet, but
rather with growth at those favorable sites chosen for subsequent investiga-
tions. Thus, deposited microorganisms must simply survive exposure to
average Martian conditions (which we already know will occur with some
likelihood), and subsequently grow upon arrival at a favorable locale.
Mission Planning and Interaction with the Soviet Spac~~g~a~m,
c / * -A.e~Y--m-- m--l-4-
We believe that the probabilities of spacecraft erosion and fracture,
survival from the ultraviolet flux, and subsequent ultimate deposition in a
warm and wet locale on Mars may be rather high. If sterilization standards
are to be relaxed, we must be quite certain - much more certain than we
can be in our present state of ignorance - that such will not be the case.
Additional considerations may be drawn concerning the reassessment
of flyby and orbiter missions to Mars. One principal contribution of the
Mariner 4 mission to these deliberations is its demonstration that a prob-
ability of 3 x 10 -5 or less for accidental planetary impact by unsterilized flyby
or orbiter spacecraft does not preclude the carrying out of useful missions.
Because of the low atmospheric pressure on Mars, spacecraft may be placed
in very close orbits permitting extremely high topographical resolution at
periapsis, without compromising the sterilization standards. The cost of any
level of commitment to prevention of contamination is much greater for
landers than for orbiters or flybys. It is also clear that any evaluation of the
risks associated with a lander mission is subject to a wider margin of error
than an orbiter mission at our present level of ignorance. This implies a
planetary exploration strategy of the following sort: Flyby and especially
orbiter missions are carried out within the present constraints on accidental
impact to obtain information that can lead to an improved assessment and
refinement of the risks of planetary landing and the parameters associated it.
At the same time, efforts to improve sterilization skills are necessary in any
case. Such remote observations from a planetary orbiter might, for example,
indicate the location and distribution of areas of increased moisture and
12
temperature (13). Whatever lander missions are carried out during this
-
period would be such that even conservative estimates of the load of terres-
trial microorganisms will be satisfied.
We now comment on the contention of Murray et al. (2) that the Soviet
-- -
space program is not failsafe in the sense that American flybys are deflected
away from the planet during midcourse maneuvers; that there is consequently
some possibility that Zond 2 impacted Mars; that future Soviet space vehicles,
undergoing sterilization procedures not evaluated in the West,will also im-
pact Mars; and therefore that the sterilization requirements on American
space vehicles can be considerably relaxed. We first point out that the
argument of (z) is inconsistent with that of (1): if there is negligible chance
of contaminating Mars, then Zond 2 has not contaminated it. But if, as we
argue, there is a significant chance of contaminating Mars, do the arguments
in (2) necessarily follow? Each contention of Murray et al. has some
- --
associated uncertainty, and it is by no means clear that any viable micro-
organisms have landed on the Martian surface as a result of the Zond mission
(22). Further information from the Soviet Union would be extremely relevant
-
in this regard. We note that an accidental impact, such as suggested for
Zond 2, interacts with the mean Martian environment, while a partially
successful lander has failure modes that permit interaction with the most
favorable environments for microbial growth on Mars. If an unsterilized
Zond 2 space vehicle has impacted Mars, it does not follow that the United
States may with abandon land spacecraft that have been only surface
sterilized. An analogy that has been useful in discussing such sterilization
is sues concerns a dry forest in tinderbox conditions. If the individual in
front of us throws a lighted match into the forest, it does not follow that we
may throw large numbers of lighted matches as well, particularly if we are
seeking out the driest parts of the forest. His match might not ignite the
forest; ours might. Also, if we are cautious with matches, the needs for
caution and the methods for achieving it might be grasped by our companion.
Our planetary quarantine program must take into account the total
history of the planet in question, and must certainly consider possible con-
tamination resulting from missions other than those of the United States. It
13
is also useful to make Soviet scientists aware that there is an interrelation-
ship between the policies of our respective nations. This interrelationship
is also "multilateral. I'
The conclusion in (2) however, that the U. S. contribution should equal
some specified fraction of the probable number of viable terrestrial micro-
organisms transferred to Mars by present and foreseeable Soviet efforts
seems to be based on some misunderstanding of the purpose of planetary
quarantine. It is not an immigration policy designed to keep the total number
of migrants matched to the resources of the target planet. It is a quarantine
program designed to minimize the possibility that the planet will be infected
with a single viable organism that might propagate now or in the future. Our
choice of quarantine policy and the costs and efforts that are a consequence
of that policy are a measure of the importance that we ascribe to an uncon-
taminated planet. Our knowledge of Soviet missions to Mars is relevant in
permitting us to estimate the probability that the planet is now infected with
viable replicating organisms. This implies some proportionate reduction in
the utility of maintaining our sterilization efforts. If we wish to keep the
ratio of the cost of our efforts to the potential gains that justify those
efforts constant, then a proportionate reduction of our sterilization efforts
could be justified. But this would then lead to a small change in the present
quarantine policy. This is a very different result from that of Murray et al.
--
They seem to argue that if there is a probability, P, that Soviet missions
have infected Mars with N viable organisms, then a satisfactory United
States policy would be to deliver some specified fraction of P x N organisms
per mission.
Keeping in mind the scientific goals of Martian exploration, and their
multilateral nature, we might conclude that these goals can be better achieved
by delivering sterilizable batteries to the Soviet Union rather than viable
microorganisms to Mars. We should make certain that specific information
about our advances in decontamination technology are widely disseminated.
There is a real possibility that a credible implementation of our present
sterilization policies will have a salutary influence in increasing the concern
14
about planetary quarantine within the Soviet Union. The proposal of Murray
et al. would have exactly the opposite effect. The COSPAR recommendations
--
of the Study Group on Standards for Spacecraft Sterilization were approved
by its parent body, the COSPAR Consultative Group on Potentially Harmful
Effects of Space Experiments and by the executive council of COSPAR. Repre-
sentatives from both the United States and the Soviet Union participated in the
activities on all levels, including those of the executive council. The Soviet
Union has also, in the recent space treaty, indicated its concern with plane-
tary quarantine. Methods should be explored by which the agreements at
COSPAR and within the space treaty can be used to increase positive feedback
on missions planning and sterilization technology between both spacefaring
nations.
15
References and Notes
1. N. H. Horowitz, R. P. Sharp, R. W. Davies, Science a, 1501 (1967).
2. B. C. Murray, M. E. Davies, P. K. Eckman, Science m, 1505 (1967).
3. C. Sagan and S. Coleman, Astronaut. Aeron. A, 22 (1965).
4. C. Sagan and S. Coleman, in Biology and the Exploration of Mars,
C. S. Pittendrigh, W. Vishniac, J. P. T. Pearman, Eds. (Nationa
Academy of Sciences, Washington, D. C., 1966) Chap. 28.
5. COSPAR Information Bull. No. 20 (NOV. 1964) p. 24.
6. R. W. Porter, private communication (1967).
7. P. K. Eckman, private communication (1967).
8. N. H. Horowitz, in Biology and the Exploration of Mars, C. S.
Pittendrigh, W. Vishniac, J. P. T. Pearman, Eds. (National Academy
of Sciences, Washington, D. C., 1966) Chap. 28.
9.
10.
11.
12.
13.
14.
15.
16.
17.
C. Leovy, paper presented at KPNO-NASA Conf. on the Atmospheres
of Venus and Mars, March (1967).
F. M. Neubauer, J. Geophys. Res. 2, 2419 (1965).
E. C. Slipher, A Photographic History of Mars (Lowell Observatory,
Flagstaff, Arizona, 1962).
J. Hotchin, P. Lorenz, C. Hemenway, paper presented at Proc.
COSPAR Space Science Collo~. , London, to be published (1967).
J. Lederberg and C. Sagan, Proc. Nat. Acad. Sci 4& 1473 (1962).
C. Sagan, J. P. Phaneuf, M. Ihnat, Icarus & 43 (1965).
W. M. Sinton, Icarus&, 222 (1967).
A. E. Ringwood, Geochim. Cosmochim. Acta 30, 41 (1966).
C. Sagan, J. B. Pollack, R. M. Goldstein, Astron. J. 72, 20 (1967);
C. Sagan and J. B. Pollack, Nature G, 117 (1967).
16
18.
19.
20.
21.
C. Sagan and J. B. Pollack, Smithsonian Astrophys. Obs. Spec. Rep.
No. 224 (1966); also submitted to J. Geophys. Res. (1967).
Dr. W. Vishniac informs us that in addition to many cases of anaerobic
osmophiles, some cases of anaerobic halophiles are in fact known - e. g.,
in the laboratory of Dr. B. Volcani, University of California, San Diego.
W. M. Sinton and J. Strong, Astrophys. J. 2, 459 (1960).
R. S. Young, P. H. Deal, 0. Whitfield, Space Life Sciences, in press
(1967); W. Jo Scott, Proc. Symp. Low Temp. Microbial. , Campbell
soup co. , Camden, N. J. p. 195 (196 1); M. F. Gunderson, ibid.,
P. 299 (1961); C. A. Hagen, E. J. Hawrylewicz, R. Ehrlich, Appl.
Microbial. u, 285 (1967); E. J. Hawrylewicz, C. A. Hagen, V. Tolkacz,
B. T. Anderson, M. Ewing, to be published (1967).
22. See, e. g., R. W. Porter, Science, in press (1967).
23. We are indebted for useful discussions to the following individuals, who
do not, however, necessarily support our conclusions: R. W. Davies,
P. K. Eckman, T. Gold, N. H. Horowitz, J. Hotchin, P. M. Hurley,
B. N. Khare, J. Lovelock, B. C. Murray, J. B. Pollack, R. W. Porter,
R. P. Sharp, W. Vishniac, and R. S. Young. This research was sup-
ported in part by NASA grants NGR 09-015-023 and NsG 81-60.
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